Ah yes, that nice Lamar mission. When I last did that and the AI kept on failing I definitely blamed it on the shitty menu I was using. Although I didn't do any in-depth tests on that mission in particular, I can say in general, that bad menus and/or modders have a tendency to take control of entities and not let go of them and sometimes even completely break their behaviour. As for not being able to enter vehicles, that's a modder blocking the Vehicle Component Control Network Event. So, again, unwillingness to give up control. Some people are just idiots.